The invention relates to a method of calibrating the original position and orientation of one or more mobile cameras and its application to the three-dimensional positional measurement of fixed objects.
It can find various applications for automatic calculation of the position of objects by taking at least one image of these objects with the camera and calculating their position according to their track on the images taken and the position of the camera.
The French patent 96 06749, not yet published, describes such an application, and the corresponding hardware system, for which the invention would be of use: it concerns the automatic recording of traffic signs along a road: one or more cameras are fixed on the roof of the vehicle, which may also be provided with an inertial navigation unit and equipment with an antenna for location with respect to a constellation of satellites in orbit (for example a GPS system); the second of these two instruments periodically (a few seconds) gives the position of the vehicle on the surface of the Earth and its course with very great accuracy, and the first one is used for updating the position and orientation information between the location times. The camera takes a series of images of each sign the vehicle passes, and an image processing system attached to the camera, which comprises means of carrying out Kalman filtering of a characteristic point of each object on each of the images, supplies coordinates representing the direction and distance of the object with respect to the camera on each of the images by means of an algorithm for predicting, checking and estimating the track of the characteristic point of the object on each image. There are also simpler algorithms, based on a triangulation from positions of characteristic points of the object on two or more images. The position of the object can therefore be determined using this information and the available knowledge concerning the position and orientation of the camera and the vehicle. The position and orientation of the vehicle are known at any instant in a fixed reference frame (related to the Earth) by virtue of the satellite location system and the inertial unit, but the position and especially the orientation of the camera with respect to the vehicle remain to be calibrated.
Two means of overcoming the uncertainties of positioning the camera on the mobile support which transports it have been proposed: assembling the camera on the support by means of a mechanical device which offers great accuracy, or providing the camera case with reference points, fixing it to the support by means of a mechanism making it possible to adjust its position, and performing a positional adjustment between the camera reference points and fixed reference points in order to align the camera viewfinder with a given direction on the support. The first possibility of this alternative requires expensive mechanical means which do not allow setting of the position of the camera on the support nor carrying out of any positional setting of any sort whatsoever, and the replacement of the camera becomes difficult or impossible since the mechanical system is adapted to each camera model. The other possibility of the alternative requires the camera to be is calibrated in the laboratory so that its reference points are tracked correctly according to the direction of the viewfinder, and precludes changing the positional settings of the camera subsequently otherwise the calibration procedure will have to be started again in order to obtain new reference points.
The advantage of the invention is essentially to escape this requirement of placing the camera on the mobile support with great accuracy in order to properly measure the position and especially the orientation of the object viewed. It also makes it possible to easily use a number of cameras on the same mobile support.
A number of advantages follow from this characteristic. It is first of all unnecessary to finely set the position at which the camera is fixed to the support, and it is possible to re-orient the camera without difficulty for a new application of the method if so desired. It is also possible, without any drawback, to mount the camera on a mobile, notably rotary, turret mounted on the support, despite the additional positional and orientational uncertainty which is then introduced; it is necessary simply to accurately measure the movements of the turret, without having to accurately know its original position and orientation; these will be determined implicitly by the method of the invention. Such a turret could for example be used in the sign recording application to facilitate the taking of successive images in places such as roundabouts where the direction of the vehicle and the field of view of the camera move very quickly.
The characteristic measurement of the invention consists in taking, for each camera, a preliminary image of a test object whose position is known in the environment; to that end, the vehicle will be moved for each camera: analysis of the image of the test object provides the position and orientation of the camera and its support with respect to the test object and therefore in a fixed reference frame. It is then sufficient to take successive images of the object which are analyzed in their turn by one of the known methods in order to know the position of the object in the fixed reference frame, by measuring the movements of the support up to each of the places where an image is taken and deducing the position of the object with the help of its track on each of the images.
It should be noted that the position and orientation of the mobile support are known at each instant including at the time the images of time test object are taken.
Where the object to be located carries a number of well identified relative position reference marks, a method of location from a single image of the object can be used.
To summarize, the invention relates in its most general form to a method of calibrating the original position and orientation of one or more cameras, disposed on a mobile support in an environment, characterised in that it consists in providing the environment with a test object at a known position and orientation; in taking an image of the test object with each camera and in analyzing the image of the test object with a processing system by reading positions of tracks of characteristic points of the test object on the image and deducing directions and distances between the characteristic points and the camera, and then a position and an orientation of the camera with respect to the test object; or, in a scarcely different design, it concerns a method of calibrating, with respect to a second camera, the original position and orientation of a first camera disposed on a mobile support in an environment, characterised in that it consists in providing the first camera with a calibration camera and the second camera with a test object at a known position and orientation; in taking an image of the test object with the calibration camera; and in analyzing the image of the test object with a processing system by reading positions of tracks of characteristic marks of the test object on the image and deducing directions and distances between the characteristic marks and the calibration camera, then a position and an orientation of the calibration camera with respect to the test object, and then of the first camera with respect to the second camera.
Advantageously, the test object comprises at least three marks appearing in the field of the camera; it has advantageously a larger number of non-coplanar marks.
The invention also relates to a method of three-dimensional positional measurement of fixed objects by means of one or more cameras mounted on a mobile support, characterised in that it consists, after having applied the calibration method, in subjecting the support to a movement up to the object while measuring this movement; in taking images of the object with the camera or cameras, between each of which the support is again moved; of analyzing the images of the object with the processing system in order to deduce therefrom a direction and a distance between the object and the camera or cameras; and finally in calculating the position of the object by using the said direction and the said distance between the object and the camera and the movements of the support.